MATLAB: If statement inside a for loop not working

for loop

Hi all,

Can someone please explain why this code is not working?

for a = 0.1:0.1:5
    if a == 3
        b = 1;
    end
end

Once 'a' becomes 3, 'b' should be assigned a 1. But nothing happens.. Not sure what is going on

Best Answer

The representation of the results of floating point maths is not 100% accurate so you should never do a straight == between a number created via floating point maths and some other number. Always test within a tolerance.

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MATLAB: Simple counter won’t trigger if statement

This is a classic beginner's problem. Your computer uses binary floating point numbers, and as such cannot exactly represent the fraction 0.1. For that reason after you have multiplied the approximation to 0.1 by ten, the result won't be an exact 1. It will probably be off in the least bit or bits. You should allow room for a small deviation from an exact 1 in your 'if' statement.

MATLAB: If statement

There are two problems with your code:

"==" doesn't work well with floating point precision (EDIT: understatement)
"if (final(1,:) == final(2,:))" should read "if all(final(1,:) == final(2,:))"

--- EDIT ---

Make sure you read the link provided by Walter and Floating points by Cleve Moler. The simple rule is: never test if two floating point values are equal.

--- EDIT ---

The internal representation and what is displayed differ. With Matlab the display format can be controlled with the function, FORMAT. With format('hex') the full internal precision is shown. Try

>> v1 = 1/3;
>> v2 = 0.333;
>> v3 = 0.3333;
>> format('hex')
>> v1
v1 =
   3fd5555555555555
>> v2
v2 =
   3fd54fdf3b645a1d
>> v3
v3 =
   3fd554c985f06f69
>> 
>> format('short')

Best Answer

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MATLAB: Simple counter won’t trigger if statement

MATLAB: If statement

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